For realization of both reduced cost and enhanced efficiency of a photoelectric conversion device, a thin film solar cell has attracted attention and been vigorously developed in recent years, since it needs fewer raw materials during production. At present, a crystalline silicon thin film solar cell on top of a conventional amorphous silicon thin film solar cell has been developed, and a stacked thin film solar cell called a hybrid solar cell obtained by stacking these cells has been put into practice. Further, studies on a compound semiconductor-based solar cell using compound semiconductor are proceeding, and a product with a higher efficiency than the thin film silicon-based solar cell has been put into practice.
The thin film silicon-based solar cell has advantages in that it is producible by a technique facilitating larger area production, such as CVD, and it has highly excellent volume cost due to raw materials being abundant. Further, although the compound semiconductor-based solar cell is inferior in volume cost to thin film silicon solar cell, it can absorb light by direct transition of electrons, thereby enhancing the efficiency with relative ease.
As for a material for the thin film silicon-based solar cell, amorphous silicon has a band gap of 1.85 to 1.7 eV. Meanwhile, crystalline silicon as a mixed phase of the amorphous silicon and crystal silicon normally has a band gap of 1.4 to 1.2 eV, depending upon a crystalline volume fraction. These thin silicon films can be alloyed with an element such as hydrogen, carbon, oxide, nitrogen or germanium, to adjust a band gap. Further, they can be doped with a material having a different valence electron number from those of silicon, such as boron or phosphorus, to obtain P-type silicon or N-type silicon.
It is to be noted that in this specification, a term “crystalline” includes polycrystalline and microcrystalline, and further includes a material partially including amorphous. Further, a term “silicon-based” includes, other than a simple silicon substance, silicon alloyed with the element such as hydrogen, carbon, oxide, nitrogen or germanium.
In the thin film silicon-based solar cell, normally, a photoelectric conversion unit is formed by a PIN structure where a substantially intrinsic I-type layer is sandwiched between a P-type layer and an N-type layer. With the I-type layer being a light-absorbing layer, a wavelength and a photovoltaic power of photoelectrically convertible light are decided according to a band gap of a material constituting the I-type layer. When energy not smaller than the band gap is absorbed, redundant energy becomes heat or light, and thus cannot be converted into electric power.
Further, it is very unlikely that energy not larger than the band gap would be absorbed. Even in the case of the energy not larger than the band gap being absorbed, electrons are not excited to a conduction band, and thus, also in this case, energy becomes heat or light, and cannot be converted into electric power. Therefore, stacking a plurality of photoelectric conversion units with different band gaps and efficiently converting light energy corresponding to the band gaps into electric power in the respective photoelectric conversion units, which is so-called multi-junction, is essential for improving the efficiency of the thin film solar cell in the future.
A compound semiconductor-based photoelectric conversion unit is an example of one expected to be stacked with the thin film silicon-based photoelectric conversion unit to form multi-junction. Although there are a variety of kinds of compound semiconductor, they are classified into three: a compound made up of a III-group element and a V-group element; a compound made up of a II-group element and a IV-group element; and further, a chalcopyrite-based compound of the I-III-VI2 group as modification of the II-VI group, or the like. Among them, CuInSe2 (hereinafter referred to as CIS) and CuInTe (hereinafter referred to as CIT), as solar cells using the chalcopyrite-based compound, have large absorption coefficients and show sufficient light absorption even with a film thickness being not larger than 1 μm.
The chalcopyrite-based compound has a band gap smaller than 1.0 eV, and excited electrons transit on the bottom of a low conductive band, whereby energy of a visible light component of solar light cannot be efficiently converted into electric power, and the compound as a simple substance is not suitable for the solar cell. For this reason, when it is used in a solar cell, a composition of chalcopyrite-based compound is changed to Cu(In, Ga)Se2 or CuIn(S, Se)2 so as to widen the band gap of the compound semiconductor. However, there is a limit on widening the band gap, and even when the composition is changed, the compound semiconductor-based photoelectric conversion unit used as a single unit cannot be suitable for the solar cell. Therefore, in order to obtain a solar cell with high practicability, a multi-junction of the compound semiconductor-based photoelectric conversion unit with other unit(s) is important.
Patent Document 1 provides a method for producing a high-efficiency solar cell in which an epitaxial growth of III-V group compound semiconductor onto a single crystal Si substrate is performed. However, forming a GaAs-based photoelectric conversion unit requires a large amount of As, and its adverse effect upon the environment is concerned. Further, since the compound semiconductor layer is required to be deposited onto the Si single crystal plane for the epitaxial growth, it is not realistic as a method for producing a large area module.
In a general chalcopyrite-based compound semiconductor-based solar cell, zinc oxide/CdS are used as a window layer on an N-type layer side. Therefore, in the chalcopyrite-based compound semiconductor-based solar cell, allowing light to be incident from the N-type layer side is one of conditions for improving the efficiency. Meanwhile, conditions for improving the efficiency of the solar cell having an amorphous silicon-based photoelectric conversion unit may include that: light is allowed to be incident from the P-type layer side of the amorphous silicon-based photoelectric conversion unit; a rate of a current of the amorphous silicon-based photoelectric conversion unit is not limited even when a multi-junction is formed; and characteristic deterioration due to photodegradation of the amorphous silicon-based photoelectric conversion unit is small.
It is expected that stacking a low-cost thin film silicon-based photoelectric conversion unit and a chalcopyrite-based compound semiconductor photoelectric conversion unit as a narrow band gap material with high sensitivity on the long wavelength side to form a multi-junction as described above can enhance the efficiency of the thin film photoelectric conversion device. However, advantages thereof cannot be sufficiently utilized when a multi-junction is formed by simply stacking them, since it is preferable to allow light to be incident from the P-type layer side on the amorphous silicon-based photoelectric conversion unit, and from the N-type layer side on the chalcopyrite-based compound semiconductor photoelectric conversion unit. Further, it is difficult to match current densities of these two photoelectric conversion units to prevent rate-limiting of the current of the amorphous silicon-based photoelectric conversion unit. There has thus been no example to date in which the amorphous silicon-based photoelectric conversion unit and the chalcopyrite-based compound semiconductor photoelectric conversion unit are stacked in a multi-junction and modularized manner.